A refractive index profile of an optical fiber is a graphical representation of the value of the refractive index as a function of optical fiber radius. Conventionally, the distance r to the center of the optical fiber is shown along the abscissa (i.e., the x axis), and the difference between the refractive index at radius r and the refractive index of the outer optical cladding of the optical fiber is shown along the ordinate axis (i.e., the y axis). The outer optical cladding has a constant refractive index and usually consists of pure silica. The outer optical cladding, however, may also contain one or more dopants. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, or “triangular” profile (e.g., an “alpha” profile) for graphs having the respective shapes of a step, a trapezoid, or a triangle. These curves are generally examples of the theoretical or set profile of the optical fiber. The manufacturing stresses of the optical fiber may lead to a slightly different profile.
An optical fiber typically includes an optical core, whose function is to transmit and possibly to amplify an optical signal, and an optical cladding, whose function is to confine the optical signal within the core. For this purpose, the refractive indexes of the core nc and the outer cladding ng are such that nc>ng. As is well known, the propagation of an optical signal in a single-mode optical fiber is divided into a fundamental mode (i.e., dominant mode) guided in the core and into secondary modes (i.e., cladding modes) guided over a certain distance in the core-cladding assembly.
As line fibers for terrestrial transmission systems, SSMF (Standard Single Mode Fiber) of dispersion-shifted fibers, also called NZDSF (Non-Zero Dispersion-Shifted Fiber), are conventionally used. Shifted dispersion fibers having non-zero and positive chromatic dispersion for the wavelength at which they are used, typically around 1550 nm, are described as NZDSF+.
Typically, the SSMFs meet specific telecommunications standards and notably the G.652 standard. The SSMFs have an attenuation of about 0.19 dB/km, measured at the wavelength of 1550 nm with a Rayleigh contribution of the order of 0.160 dB/km, an effective area of about 80 μm2, an effective cut-off wavelength less than 1350 nm, a positive chromatic dispersion of about 17 ps/(nm·km) at 1550 nm and a positive dispersion slope of 0.058 ps/(nm2·km).
The NZDSF+ fibers, at the wavelength of 1550 nm, have a lower chromatic dispersion than SSMFs, typically between 3 and 14 ps/(nm·km), and a chromatic dispersion slope typically less than 0.1 ps/(nm2·km). The NZDSF+ fibers are generally used for short distance transmission systems and meet specific telecommunications standards, notably the G.655 and G.656 standards.
FIG. 1 shows the set profiles of an SSMF and of a standard NZDSF. The illustrated profiles are set profiles (i.e., representative of the theoretical profile of the fiber). Those having ordinary skill in the art will appreciate that the fiber actually obtained after drawing a fiber from a preform may have a slightly different profile.
Typically, an SSMF includes a central core with a radius of 4.35 μm and having an index difference of 5.2×10−3 with the outer cladding acting as an optical cladding. A standard NZDSF includes a central core having an index difference Dn1 with an outer cladding, acting as an optical cladding, an intermediate cladding having an index difference Dn2 with the outer cladding, and a ring having an index difference Dn3 with the outer cladding. The refractive indexes in the central core, in the intermediate cladding, and in the ring are substantially constant over all their widths. The width of the core is defined by its radius r1. The widths of the intermediate cladding and the ring are defined by their respective outer radii, r2 and r3. Typically, the central core, the intermediate cladding, the ring, and the outer cladding are obtained by CVD-type deposition in a silica tube and the optical cladding is formed by the tube and the overclad of the tube, generally in natural or doped silica, but it may also be obtained by any other deposition technique (VAD or OVD).
As illustrated in FIG. 1, the NZDSFs have a central core with a smaller radius and a larger index difference than the central core of an SSMF. With this core dimensions, chromatic dispersion may be reduced. The more significant doping of the core as compared with an SSMF, however, introduces more significant Rayleigh scattering losses, larger than 0.164 dB/km leading to an attenuation larger than 0.190 dB/km at 1550 nm.
It is desired to be able to reduce the attenuation of an NZDSF to a value equivalent to that of an SSMF. The attenuation in an optical fiber is due mostly to Rayleigh scattering losses and partly to absorption losses and to losses due to defects of the guide.
In the case of an NZDSF, the presence of dopants in the core in a higher concentration as compared with an SSMF, increases losses by Rayleigh scattering. It is known to reduce Rayleigh scattering losses by making fibers with a pure silica core. This for example is what is proposed in the publication “Ultra Low Loss (0.1484 dB/km) Pure Silica Core Fiber” of K. Nagayama et al., published in SEI Technical Review, No. 57, January 2004; or in the publication “Optical Loss Property of Silica-Based Single Mode Fibers” of M. Ohashi et al., published in the Journal of Lightwave Technology, Vol. 10, No. 5, May 1992, pp. 539-543. The fibers with a pure silica core, however, are costly to manufacture because of the obligation of burying the optical cladding by doping (e.g., with fluorine).
It is also known to reduce losses by Rayleigh scattering by optimizing the fiber-drawing conditions. This for example is what is described in the publication “Rayleigh Scattering Reduction Method for Silica-Based Optical Fiber” of K. Tsujikawa et al., published in the Journal of Lightwave Technology, Vol. 18, No. 11, November 2000, pp 1528-1532; or in the publication “A high performance GeO2/SiO2 NZ-DSF and the prospects for future improvement using Holey Fiber technology” of K. Mukasa et al., published in ECOC'05, Tu 1.4.6. The proposed solutions are complex to apply industrially, however, because several fiber-drawing temperatures are used with heating and cooling cycles, which are difficult to control.
U.S. Pat. No. 6,576,164, which is hereby incorporated by reference in its entirety, discloses a method for making SSMF wherein the fiber-drawing conditions are optimized in order to reduce the losses by Rayleigh scattering. The method proposed in this document however requires complex equipment with additional cooling devices.
European Patent No. 1,256,554, and its counterpart U.S. Pat. No. 6,904,213, which is hereby incorporated by reference in its entirety, discloses a method for making a step-index fiber comprising a germanium-doped central core and outer and optical claddings with an index less than that of silica. Because the cladding is partly buried, the amount of dopant in the core may be reduced and attenuation in the fiber is reduced. Such a solution is however costly and not directly applicable to an NZDSF type fiber.
European Patent No. 1,288,685, and its counterpart U.S. Pat. No. 6,819,850, which is hereby incorporated by reference in its entirety, discloses a non-zero dispersion-shifted fiber comprising a central core, an intermediate cladding, a ring, a depressed cladding and an optical cladding. Rayleigh losses are not mentioned.
European Patent No. 1,434,071 and its counterpart U.S. Pat. No. 7,171,092, European Patent No. 1,382,981 and its counterpart U.S. Pat. No. 6,928,222, European Patent No. 1,734,390 and its counterpart U.S. Pat. No. 7,428,361, and European Patent No. 1,865,348, each of which is hereby incorporated by reference in its entirety, disclose a dispersion compensating fiber having a central core, an intermediate cladding, a ring, a depressed cladding, and an optical cladding. Rayleigh losses are not mentioned.
European Patent No. 1,610,160, and its counterpart U.S. Pat. No. 7,327,921, which is hereby incorporated by reference in its entirety, discloses a dispersion compensating fiber comprising a central core and at least five inner claddings. Rayleigh losses are not mentioned.
FIG. 2 shows a set profile of an NZDSF for which the whole of the structure would have been partly buried, i.e., lesser doping of the core and with an intermediate cladding and outer cladding having smaller indexes than that of silica. The optical cladding (which can be made by OVD, VAD, CVD, by the tube or by the overclad of the tube in which the fiber preform is made) is maintained in silica for reasons of costs. If such a fiber profile actually reduces the losses by Rayleigh scattering to a value substantially equal to that of an SSMF, the bending losses are very clearly degraded (see Example 2a of the Tables I and II below).
U.S. Pat. No. 4,852,968, which is hereby incorporated by reference in its entirety, discloses that with a buried trench it is possible to reduce the bending losses. However, by simply adding a buried trench to the structure proposed in FIG. 2, it is not possible to reach acceptable bending losses as this is shown by Example 2b of the Tables I and II (below).
Therefore, there exists a need for an NZDSF+ fiber having reduced Rayleigh scattering losses without degradation of the other optical parameters, notably bending losses, and which can be manufactured at reasonable cost and without changing the fiber drawing equipment.